"Trump scares me," Cuban said after speaking on behalf of Clinton and Tim Kaine, her vice presidential nominee. "Donald, initially, I really hoped he would be something different, that as a businessperson, I thought there was an opportunity there. But then he went off the reservation and went bats--- crazy."

Cuban also said that he had talked to Trump before he endorsed Clinton and urged the real estate mogul-turned-candidate that at some point he had to adapt and learn policy.

"I told him directly, at some point you have to learn, at some point you have to read and that that job is all about uncertainty and if you don't do the work, that is not good for this country," Cuban said. "And he just doesn't do the work."

In endorsing Clinton, Cuban said, "I am ready to vote for a true leader, I am ready to vote for the American Dream. I am ready to tell the world that I am here to endorse Hillary Clinton."

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Earlier in the evening, Cuban delivered multiple crowd-pleasing zingers at Trump, referring to the Republican presidential candidate as a "jagoff" in a colorful speech.

"You know what we call a person like that -- the screamers, the yellers, the people that try to intimidate you? You know what we call a person like that in Pittsburgh? A jagoff," said Cuban, a Pittsburgh native. "Is there any bigger jagoff in the world than Donald Trump?"

Cuban, a judge on the reality TV show "Shark Tank," even mocked "The Apprentice," which Trump previously hosted, saying his show "kicked 'The Apprentice's' ass."

But beyond the jabs at Trump, Cuban's broader message to voters here was that Clinton has the qualifications and leadership skills to help preserve the American dream.

"In Hillary Clinton's America, the American Dream is alive and well," he said. "There is no place that knows that better than Pittsburgh because we are an American Dream city."

Cuban's endorsement comes in the middle of the Clinton-Kaine campaign's three-day bus tour through the Rust Belt, where they are selling their economic vision and plan to working class Americans. After traveling west through Pennsylvania, they will next head to Ohio, campaigning in Youngstown and Columbus.

According to a campaign aide, Cuban and Clinton campaign chairman John Podesta spoke multiple times throughout the 2016 campaign, and met in the Brooklyn campaign headquarters a few weeks ago. Cuban reached out to Podesta Thursday night to signal that he was ready to make an endorsement.

Clinton thanked Cuban for his endorsement after the introduction, referencing the reality TV star throughout her speech.

"I can't tell you how happy I am to be here with a Pittsburgh native, who worked hard for everything that he has made and has helped a lot of people along the way what he exemplifies is the American dream," she said of Cuban.

Clinton also name-checked Cuban while speaking about her tax plan. When she pledged to make "those who have been successful" pay their fair share, she turned to the billionaire and said, "Sorry Mark."